Valve is quite clear about their reasoning. Since AI models use all kind of sources for their training, they don't want those assets on their platform because they are afraid of copyright claims.
For all we know, the game in question had images clearly aping some licensed characters. We don't know how stringent the policies are without clarification or examples of art found infringing. How did Valve know that the art was AI-generated? Did the developer tell them or include it in their marketing materials? It's basically just reading tea leaves without that information.
It actually sounds like if you claim to have ownership of the training data you can still use AI generated assets. For most people this is a distinction without a difference however.